1. Field of the Invention
This invention relates to the field of stereolithography.
2. Discussion of Prior Art
Stereolithography is a process by which highly accurate solidified articles can be formed from liquid photosensitive polymer resin. When exposed to ultra violet radiation, the photosensitive resin cures to form a solid plastics material.
In a known stereolithographic process, a fine, intense ultra violet laser beam is directed on to the liquid surface of the resin. Where the beam makes contact with the resin, the resin cures to a depth of typically 0.15 mm. To allow an article to be formed by curing the resin, it is necessary to split the article into very thin sections, each of approximately 0.1 mm thickness. A movable perforated tray is placed in the resin, initially at a depth of approx 0.1 mm below the surface of the resin. The laser beam is then moved across the surface of the resin to form a solidified trail, representing a cross section of the article, leaving adjacent areas liquid. When this section of the article is completed, the perforated tray is then dipped lower into the resin allowing resin to flow through the perforations and to form a fresh layer of liquid resin which coats the upper surface of the previously solidified layer. The tray is then raised to be positioned approximately 0.1 mm deeper into the resin than for the previous section. After the resin has settled back to a flat surface, the laser beam is again directed over the liquid surface of the resin to form a second solidified cross section of the article. As the laser beam is curing the resin to form this cross section of the article it also cures this cross section onto the previous one, so that the two sections are integrated. The tray is then dipped into the resin again and positioned at a level of 0.1 mm below its position for the previous section. The surface of the resin is again cured by the laser beam and simultaneously integrated with the section below, and in this manner the article is gradually formed from the liquid resin.
This method has the advantage that the cross sections of the article are simultaneously integrated as the resin is cured and so there is no need to bond individual parts to form an article and hence there is no weak bond line which can affect the structural performance of the article. The article demonstrates the structural qualities it would exhibit if it were formed from one section rather than several bonded together.
Articles can be made by creating solid layers, which result in dense, solid articles. Alternatively articles can be made by creating less dense cross-hatched layers which result in articles with a honeycomb-like structure, known as xe2x80x9cquickcastxe2x80x9d articles. Quickcast articles have a structure comprising considerably more air than resin and are therefore lighter and cheaper, as much less resin is used in their manufacture. Furthermore, as less of the resin is cured during the manufacture of a quickcast article, the time taken to manufacture a quickcast article is considerably shorter than the time taken to manufacture a comparable solid article.
Software packages are available which take the design data for the article (such as a three-dimensional Computer Aided Design (CAD) model) and use the data to define cross-sectional slices of the article of approximately 0.1 mm thickness. The design information for each layer of the article may then be fed into a program which controls the movement and position of the laser beam and the tray located in the resin, so that articles can be built up accurately from the design data cross sections. Such a package which includes the slicing of the CAD data and the control of the stereolithography apparatus (including the laser beam and the tray) is commercially available under licence from 3D Systems, for example.
It is often desirable to manufacture an article which is comprised of both dense and less dense regions. An example of such an article is a model used for windtunnel testing which is required to be highly accurate in its dimensions. Such models made by traditional machining methods tend to be extremely expensive and it is hard to achieve the desired accuracy. Stereolithography provides a method of manufacture of extremely accurate articles meeting the strict requirements for windtunnel models. However, producing solid models by stereolithography is time consuming and is as expensive as traditional manufacture due to the high price of the resin. Producing these models by the alternative quickcast method can reduce the cost by approximately 50% and the time for manufacture by approximately 30% whilst maintaining the high standard of accuracy demanded. It is therefore desirable to produce such a model as a quickcast article rather than a solid one.
In some applications, such as during wind tunnel trials for example, it may be necessary to attach the model to some fixed object. The securing of models used in wind tunnel trials is traditionally achieved by drilling a hole into the rear or underside part of the model and attaching the model to a stand fixed in the wind tunnel.
However, quickcast articles cannot be drilled to allow attachment to wind tunnel stands or other articles. The material may shatter if an attempt is made to drill into it or to machine it. Solid articles, however, may be drilled into without a substantial risk of shattering.
No satisfactory method of providing a quickcast model with a solid part suitable for accurate drilling and attachment has hitherto been devised.
To produce an article having a less dense region and a denser, solid region, it has until now been necessary to produce separate components each having either less dense or dense, solid properties and then to bond these together using an adhesive resin. This produces an article which has a discernible bond line, which gives an overall weak structure to the product. In the case of models for wind tunnels, any models being made from several components bonded together are not of a sufficient strength to withstand the rigours of wind tunnel testing. They may bend at the bond line, so distorting the shape of the model under test, or may actually break at these weaker points. This structural behaviour is clearly unacceptable in any applications where the stereolithographic articles will face high stresses.
The present invention seeks to provide a method for producing a stereolithographic article, which aims to overcome the above problems associated with the adhesive bonding of less dense and denser, solid regions to make known articles.
According to the present invention there is provided a method for manufacturing a stereolithographic article comprising a plurality of components, said article having a unitary nature and at least two of the components having different densities, said method comprising the steps of:
Producing design data for said article;
Separating the design data for the article into regions of different densities, these regions forming individual components;
Loading the design data for said individual components into a software package adapted to slice the design data into thin cross sectional layers;
Positioning said individual components using said software package to form a substantially complete and accurate representation of said article, said software package enabling a user to position a number of said components within a volume, said volume representing a volume of resin from which said article is to be manufactured;
Slicing said design data for said individual components, said components being positioned to form a substantially complete and accurate representation of said article, into thin cross sectional layers;
Feeding the design data for each cross sectional layer of each of said components into a control program, said control program being adapted to control a stereolithographic apparatus comprising a laser beam and photosensitive polymer resin;
Operating said control program to produce a substantially unitary article from said components.
The components are preferably enlarged by an amount sufficient to enable an overlap with adjacent components, when positioned in said volume using said software, said overlap being at least 0.05 mm.
The overlap between adjacent components is advantageously 0.1 mm.
The article is preferably substantially of the required accuracy and size when the components are positioned to overlap adjacent components.
The cross sectional layers are preferably in the range 0.05 to 0.2 mm thick. Advantageously the cross sectional layers are 0.1 mm thick.
The unitary article produced preferably has substantially no structural joint between regions of different density. The resin used in this process is preferably liquid at room temperature and pressure and cures and solidifies on contact with ultra violet radiation.